Secondary batteries, which generate electrical energy by way of electrochemical oxidation and reduction reaction, are used over a wide range of applications. For example, the batteries are being used in increasing range of areas including portable apparatuses such as portable phone, laptop computer, digital camera, video camera, tablet computer, power tool, gearing tool and so on that can be carried around in a user's hand; various electrically-driven power apparatuses such as electric bicycle, electric motorcycle, electric vehicle, hybrid vehicle, electric ship, electric airplane, and so on; a power storage apparatus being used to store power generated through new renewable energy or surplus power generated; and an uninterrupted power supply apparatus and so on for stably supplying power to various information communication apparatuses including server computers and base stations for communication.
The batteries includes three basic elements which are: an anode containing a material that undergoes oxidation and releases electrons during discharging; a cathode containing a material that undergoes reduction and accepts electrons during discharging; and an electrolyte that allows migration of ions between the anode and the cathode. The batteries can be categorized into primary batteries which are not reusable once they are discharged, and secondary batteries which have at least partially reversible electrochemical reaction, thus are repetitively chargeable and dischargeable.
Among these, as the secondary batteries, lead-acid battery, nickel-cadmium battery, nickel-zinc battery, nickel-iron battery, silver oxide battery, nickel metal hydride battery, zinc-manganese oxide battery, zinc-bromide battery, metal-air battery, lithium secondary battery, and so on, are known. Among these, the lithium secondary batteries are attracting greatest commercial attentions, in view of their relatively higher energy density, higher battery voltage and longer storage life than other secondary batteries.
Meanwhile, an electronic apparatus where the secondary battery is applied generally has a function of providing information on the remaining usable amount based on a state of charge (SOC) of the secondary battery, and such an SOC of the secondary battery is usually obtained according to a profile of an aspect of change of the SOC caused by a change in an open circuit voltage (OCV).
Such a profile of the aspect of change of SOC-OCV differs not only depending on the type or capacity and so on of a subject secondary battery being applied, but also differs as degradation by use progresses even when the type or capacity and so on of the secondary battery is specified.
In order to predict the SOC with precision using such a profile of an aspect of change of SOC-OCV, it is necessary to measure the OCV of the secondary battery with precision, and since the OCV can be measured with precision in a state where the secondary battery is completely stabilized, it is difficult to quickly measure with precision the OCV that changes constantly as the secondary battery is used.
Therefore, the need arises to study ways to easily predict an SOC-OCV profile after a certain level of degradation has progressed using a profile of an aspect of change of an SOC-OCV pre-measured according to the type or capacity and so on of the secondary battery being applied, that is, a known SOC-OCV profile of a fresh cell.